/*
7-137 Average
分数 10
作者 翁恺
单位 浙江大学

You are going to read a serial of none-negative integers, which ends with a negative number that does not count as one of the data. Your program calculates the average of all the none-negative integers, and prints the average which rounds to two decimal places.

When there's no valid data at all, your program prints out:

None
输入格式：

A serial of none-negative integers, ending with a negative number, which is not one of the valide data.
输出格式：

A number rounds to two decimal places, which is the average of the serial.

The printf for this case is:

printf("%.2f\n", (double)sum/count);
输入例子

1 2 3 4 5 6 -1

输出例子

3.50
*/
#include <stdio.h>
int main() {
    double sum = 0;
    int count = 0;
    for (;;) {
        int v;
        scanf("%d", &v);
        if (v < 0) {
            break;
        }
        sum += v;
        count++;
    }
    if (count > 0) {
        printf("%.2f\n", sum/count);
    } else {
        printf("None");
    }
    return 0;
}